This invention relates to an orthopaedic implant intended for replacing a portion of a bone. More particularly, it relates to a means for augmenting such an implant so as to adapt the implant to fit a variety of situations.
In order to replace a defective portion of a bone, such as an arthritic joint, an orthopaedic surgeon cuts away the defective portion and shapes the remaining bone to a specific geometry. An implant, having bone contacting surfaces corresponding to this specific geometry, is then placed on the bone. The implant is typically either provided with a porous surface for the bone to attach to or it is cemented onto the bone.
Implants of this type, for a particular skeletal joint, are usually made available in a range of sizes to fit the varying sizes in the patient population. However, only a relatively small number of different sizes can practically be made available. Therefore, difficulties arise when a patient has a bone of an odd size or when the bone is abnormally shaped or deformed due to disease. Often, when a bone is shaped to the specific geometry to fit an implant, only a small area of the prepared bone will exhibit a deformity such as insufficient bone to support the implant. In such cases, implant augments have been successfully used to supplement the implant and fill the bone deficiency. These augments comprise small blocks or wedges of material that can be affixed to the implant. While augments have been generally successful, the various means of attaching them to the implant have been problematic. Prior augments have been attached using screws, cements and clips.
The challenge is to provide a strong, stable attachment with a minimum number of fastening elements that are easily employed. The attachment must resist linear as well as rotational forces and large separation forces as well as smaller forces causing slight relative motion between the augment and the implant. This slight relative motion, often referred to as micro-motion is problematic in that it can lead to wear debris from the augment and implant and it can prevent the growth of a solid bony interface adjacent the augment and implant.